Keyless chucks are known which comprise a base body having a central longitudinal axis and connected for rotating with a drive shaft, and an outer casing coupled outside the base body such that it can rotate coaxially but not move axially with respect to the base body. The outer casing has an axial opening, and a plurality of jaws are arranged movably with respect to the outer casing and communicated with the axial opening. A pusher is coupled to the base body by a threaded coupling and furthermore coupled to the outer casing such that it can slide axially but not rotate coaxially. The pusher is operatively coupled with the jaws, such that rotation of the pusher with respect to the base body in a tightening direction moves the jaws towards the central longitudinal axis to grip a tool, and rotation of the pusher with respect to the base body in a opposite loosening direction moves the jaws away from the central longitudinal axis to release the tool.
However, when keyless chucks of this type are used in machines or motor-driven power tools with a reversible rotation direction or in hand-powered tools, there is a risk that the jaws will loosen in an unwanted manner if the machine, the power tool or the hand-powered tool is operatively driven in a reverse rotation direction coinciding with the mentioned loosening direction for loosening the jaws.
To prevent this risk, some known keyless chucks incorporate a locking device that allows locking the jaws in a gripping position. For example, patent document U.S. Pat. No. 8,403,339 B2 discloses a keyless chuck of the type described above including a locking device comprising a toothed ring that is fixed with respect to the base body and provided with a plurality of asymmetrical teeth having a saw tooth profile, a locking ring coupled outside the outer casing such that it can rotate but not move axially with respect to the outer casing, and a ratchet ring moving axially with respect to the outer casing and with respect to the locking ring between axial locking and unlocking positions when the locking ring is rotated coaxially with respect to the base body in cooperation with a cam and an elastic element. The ratchet ring has a plurality of asymmetrical ratchet teeth opposite and conjugated with the asymmetrical teeth of the toothed ring. When the ratchet ring is in the axial locking position, the ratchet teeth are coupled with the teeth of the toothed ring, blocking relative rotation between the base body and the outer casing in a loosening direction for loosening the grip of the jaws but allowing, by virtue of the asymmetrical profile of the teeth and of the elastic element, relative rotation in a tightening direction for tightening the grip of the jaws, and when the ratchet ring is in the axial release position, the ratchet teeth are separated and free of contact with the toothed ring.
In said patent document U.S. Pat. No. 8,403,339 B2, the elastic element is arranged such that it permanently pushes the ratchet ring towards the axial locking position and the cam moves the ratchet ring towards the axial unlocking position in opposition to the force exerted by the elastic element.
Utility model document ES 1072879 U discloses a chuck including a hybrid-type fitting mechanism combining a self-tightening mechanism with a key mechanism. Nevertheless, this hybrid-type chuck does not include a locking mechanism.